One of the goals in production tool making is a tool that will be both hard enough to stand up under service conditions and tough enough not to crack, either in manufacture or in use.

Five factors contribute to making a successful tool: good design, steel of the proper grade, correct heat treatment, proper grinding and proper use of the tool.

The basic requirements for hardening any of the standard types of tool steel are heating, cooling and re-heating. This includes heating the steel to a sufficiently high temperature to make the necessary change in structure, followed by cooling at a rate fast enough to develop the desired hardness. Tempering follows immediately after quenching to remove stresses induced by the quenching process.

Steel undergoes definite internal changes when subjected to a temperature above its critical range. After reaching this temperature, if the steel is allowed to cool naturally, it will return to a normal condition.

For steel to return to its normal condition after being heated above its critical range, sufficient time must elapse during the cooling so that the internal changes which took place during heating will have time to reverse themselves.

If the heated steel is cooled before these internal changes reverse, certain modifications of the structure will be fixed in the steel which will alter the physical characteristics of the steel, tensile strength, hardness, toughness, etc.